He didn't.... there's a quote from John in the "Anthology" book, regarding "Maxwell's Silver Hammer", and he mentions his not being on it because he was ill after that car accident he had around that time...

... and goes on to say that, on that one, Paul had really "ground George and Ringo into the ground"

Thanks for the confirmation.

Incidentally John and Yoko did their own granny music towards the end. "Yes I'm Your Angel" and "Grow Old With Me" were much worse than Maxwell imho.